<!DOCTYPE html>
<html>
<body>
<form id="myForm">
<fieldset>
<legend id="myLegend">Personalia:</legend>
Name: <input type="text"><br>
Email: <input type="text">
</fieldset>
</form>
<p>Click the "Try it" button to return the id of the form the legend belongs to.</p>
<button onclick="myFunction()">Try it</button>
<p id="demo"></p>
<script>
function myFunction() {
var x = document.getElementById("myLegend").form.id;
document.getElementById("demo").innerHTML = x;
}
</script>
</body>
</html>